ABSTRACT

Proactive and transparent decision-making about the long-term management of dams is critical for successfully managing this aging infrastructure and presents an opportunity to weigh the many services and disservices dams provide. Many tools support structured decision-making about the removal of dams at watershed-scales. However, these tools may constrain decision processes by omitting important user objectives. We conducted a literature review to define objectives of dam removal decisions and found that common objectives like reducing safety hazard and expanding recreational opportunities were included in few of the 41 existing decision-support tools we reviewed. Most tools only included two or three of the 18 potential dam removal objectives that we identified. To facilitate transparent deliberation of diverse objectives and their incorporation into decision processes, we developed a web application which guides users to identify their objectives, select metrics and methods to evaluate management alternatives, and identify appropriate decision tools to weigh multiple objectives. We demonstrate this web application as a resource for the dam management community of practice with a case study about the decision to remove a dam in Athens, GA, USA. More broadly, we propose the process outlined here as a model for aligning diverse objectives in other types of river infrastructure decisions. Given the contribution of this infrastructure to declining biodiversity, intensifying climate, and development needs, failing to align multiple objectives in river infrastructure decisions can represent consequential missed opportunities.

INTRODUCTION

Dam infrastructure has been critical to global economic development, but many dams and their reservoirs have exceeded their design lifetimes and are no longer fulfilling their constructed purposes (Gonzales & Walls, 2020; Perera et al., 2021). In the United States, decades of disinvestment in maintenance has also led to significant deficiencies, such that the American Society of Civil Engineers rated dams as one of the worst-performing categories of infrastructure (ASCE, 2021). The estimated cost to repair all known deficient, non-federal dams in the US National Inventory is $76 billion (ASDSO, 2022); however, this estimate includes only a fraction of the total number of dams in the US. For many deficient dams, removal may be a more economically efficient alternative to repair, particularly when the dam no longer serves its constructed purpose (Doyle, Stanley, et al., 2003; Grabowski et al., 2018; IEC, 2015). Even for functioning dams, removal may be the best long-term management strategy due to benefits for wildlife, water quality, recreation, safety, flood risk mitigation, and other services (Hansen et al., 2020).

The decision of whether to remove a dam, or which dams to remove, requires weighing the many services and disservices provided by a dammed versus a free-flowing stream, which can vary greatly according to local context (Habel et al., 2020). Additionally, dam decommissioning decisions encompass multiple scales of complexity including variation in the number and spatial relationship of dams considered for removal, dam size and impounded volume, diverse ownership, relevant legal authorities, and administrative frameworks (McKay et al., 2020; USSD, 2015). In part due to this complexity, most dams have been removed in an ad hoc manner despite calls for coordinated and structured decision processes (Doyle, Harbor, et al., 2003; Neeson et al., 2015). However, recent large infrastructure investments, such as the $2.4 billion allocated for dam decommissioning, rehabilitation, and retrofit in the 2021 U.S. Infrastructure Investment and Jobs Act (Pub. L. 117-58), may open a policy window for more coordinated dam removal. Several organizations have developed tools to support structured decision-making about dam removals, but existing tools may constrain options or lead to suboptimal outcomes if they do not include all relevant objectives of concern. Identifying these objectives is a critical early step in structured decision-making (Gregory & Keeney, 2002). To facilitate decision-making inclusive of diverse objectives, we (1) compiled a list of objectives relevant to dam removal decisions, (2) examined their representation in existing dam removal decision-support tools which we identified in a literature search (n = 41, see below), (3) developed a new web application that links objectives to relevant metrics, methods, data sources, and decision support tools, and (4) applied this web application to a decision about whether to remove dam.

DAM REMOVAL OBJECTIVES AND THEIR REPRESENTATION IN DECISION-SUPPORT TOOLS

To identify common objectives in dam removal decisions and evaluate their inclusion in existing decision-support tools, we performed a two-stage search of the literature. We first conducted a non-systematic, preliminary literature review to develop an initial list of objectives of dam removal decisions. These objectives included motivations for dam removal (e.g., maximize swift water recreational opportunities) as well as services to be maintained regardless of the removal decision (e.g., meet navigational demands). We then refined this list as we conducted a second, systematic review of existing decision-support tools published before April 2022 to determine how frequently our identified objectives were included. Of the 108 evaluated tools and papers, 41 met our inclusion criteria (additional details in Supporting Information). In total, we identified 18 common objectives of dam removal, which we organized into five categories (Table 1).

Table 1. Common objectives of dam removal identified by a literature review.

Category	Objective
1. Account for monetary costs and feasibility	1a. Minimize implementation costs
	1b. Minimize maintenance costs
2. Meet demands for infrastructure services	2a. Meet water demands
	2b. Meet power generation demands
	2c. Meet navigation demands
	2d. Reduce flood risk
3. Reduce safety hazard	3a. Reduce personal safety risk
	3b. Mitigate risk of failure
4. Meet community desires for use of rivers for recreation, historic preservation, and sense of place	4a. Maximize swift water recreational opportunities
	4b. Maximize flat water recreational opportunities
	4c. Minimize adverse impacts to sites judged by the community, state, or Nation to be historically significant places
	4d. Minimize interruption to community sense of place
5. Maintain and restore the physical, chemical, and biological integrity of the Nation’s waters	5a. Promote/maintain a population or community of focal taxa
	5b. Promote/maintain biodiversity
	5c. Prevent the spread of invasive species, disease, or undesirable hybridization
	5d. Reduce greenhouse gas emissions
	5e. Maintain or improve water quality
	5f. Reduce stream geomorphic degradation

Out of 41 dam removal decision-support tools, only ten included more than three objectives. The tool with the greatest number of objectives included 9 of the 18 we identified (Brown et al., 2009). Supporting/maintaining a population or community of focal taxa was the most frequently included objective (33/41 tools), followed by minimizing implementation cost (29/41 tools) (Figure 1). Unsurprisingly, these objectives were the most frequently co-occurring, followed by the combination of supporting/maintaining a population or community of focal taxa and meeting power generation needs (Figure 2). No tool included the objective reduce stream geomorphic degradation. The objectives reduce personal safety risk, maximize swift water recreational opportunities, maximize flat water recreational opportunities, minimize adverse impacts to sites judged by the community, state or nation to be historically significant places were each represented in only one tool. Because these objectives were poorly represented in existing decision-support tools, we identified them as high priorities for metric development. We anticipated that developing metrics for these objectives would facilitate their inclusion in structured decision-making for evaluating management alternatives.

Figure 1. Frequency of A) occurrence and B) co-occurrence of identified objectives among existing dam removal decision-support tools (n = 41).

IDENTIFYING METRICS AND METHODS FOR QUANTIFYING OUTCOMES

Metrics operationalize objectives by describing the extent to which a decision alternative achieves an objective (Keeney & Gregory, 2005; McKay et al., 2012). Metrics can be quantified using different methods. For example, the accessibility of the river network to an organism, often called network connectivity (a metric associated with Objective 5a), can be calculated assuming that dams are impassable barriers. Alternatively, connectivity can be calculated assuming that part of the population (0-100%) can traverse the dam (McKay et al., 2017). To varying degrees, metrics abstract the processes which generate an outcome (i.e., the achievement of an objective) from an action (e.g., dam removal). If a metric substantially mischaracterizes this process, using it may lead to suboptimal decisions (Keeney & Gregory, 2005). For example, assigning higher scores to alternatives that yield greater river network connectivity assumes that dam removal will have a positive impact on the focal population by providing greater access to the river network. Connectivity metrics will fail to capture this objective if the organism is more constrained by factors other than connectivity, such as unsuitable water temperature or nutrient pollution (Reid et al., 2019).

We compiled metrics and methods associated with our 18 identified objectives from the literature and developed additional metrics and methods where we identified gaps (Supporting Information). We organized these linked objectives, metrics, and methods along with their required data sources in an interactive web application called the Dam Objectives & Metrics Selector Application (https://lnaslund.shinyapps.io/MCDA/) (Figure 2, more details in Supporting Information). This web application can be used to populate different frameworks for evaluating tradeoffs among proposed alternatives, and the application lists decision support tools that include the user’s selected objectives. We see the application as a complementary rather than a stand-alone tool to support critical thinking about the objectives of dam management and to guide users to resources that align with their objectives. For example, an analyst who wishes to employ optimization methods could use the tool to identify computationally efficient metrics to parameterize their selected objectives and constraints. An analyst who wishes to employ scoring and ranking methods could use the tool to identify metrics for parameterizing objectives that are difficult to quantify like minimizing interruption to community sense of place (Objective 4d). The tool is intended to support decisions about single as well as multiple dams. We provide an example for a single dam decision and support for multiple dam decisions below.

Figure 2. The Dam Objectives & Metrics Selector web application. A) The user selects objectives of dam removal from the objectives tab using the information provided in the grey buttons organized by objective category. B) The metrics tab displays metrics and methods associated with the selected objectives and provides additional information, including citations and data sources in the information buttons. C) The tools tab displays the tools in the literature review (41 tools total) which include the user’s selected objectives.

DECISION SUPPORT IN A SINGLE DAM REMOVAL (WHITE DAM, ATHENS, GA, USA)

We illustrate the utility of the Dam Objectives and Metrics Selector Application for evaluating decisions about a single dam using a post-hoc evaluation of the White Dam in Athens, Georgia, USA, which was partially removed in 2018 (Figure 3). The White Dam was a concrete gravity dam which spanned the Middle Oconee River. It was constructed in 1913 for hydropower and was in operation until the 1950s. In 1978, the dam was acquired by the University of Georgia as part of a land donation (The Georgia Aquatic Connectivity Team, 2020). In 2014, university staff proposed the removal of White Dam to reduce liability concerns from hazards to recreational safety (Objective 3a) caused by large wood jams that occasionally formed at the structure (Figure 3) (The Georgia Aquatic Connectivity Team, 2020). The project team considered four management alternatives: 1) do nothing, 2) completely remove the dam, 3) leave the dam in place and construct a bypass channel, 4) remove the center section of the dam wall but leave the remaining structure in place (partial removal) (The Georgia Aquatic Connectivity Team, 2020).

The web application provides a platform to systematically consider the relevance of objectives to a removal decision (Table 2). When the relevance of an objective is unknown, the metrics and methods in the application may be used to identify ways to determine its relevance (Table 2). Using the application, we identified five relevant objectives and selected appropriate metrics and methods (Table 2). For the four objectives with unknown relevance, we identified metrics and methods from the application to evaluate their inclusion in the decision process (Table 2). As objectives are determined to be relevant by this evaluation, the process of considering tradeoffs among alternatives may be iterated.

Table 2. Systematic consideration of the relevance of the objectives in the Dam Objectives and Metrics Selector tool to the White Dam Removal, Middle Oconee River, Athens, GA, USA. For relevant objectives, a metric/method is provided to evaluate alternatives according to their achievement of the objective. For objectives with unknown relevance, a metric/method is provided to help determine their relevance.

Objective	Relevance	Justification	Metric: Method
1a. Minimize implementation costs	Relevant	The dam owners had additional funding priorities.	Cost of removing the dam and appurtenant structures or constructing the by-pass channel: consult relevant experts
1b. Minimize maintenance costs	Not relevant	The dam provided no infrastructure services to supplant.
2a. Meet water demands	Not relevant	No water intakes were served by the dam.
2b. Meet power generation demands	Not relevant	Power production potential was determined to be uneconomical in the 1980s.
2c. Meet navigation demands	Not relevant	The river does not support commercial navigation and the dam did not support terrestrial navigation.
2d. Reduce flood risk	Not relevant	The dam did not have the capacity to impound substantial volumes of water.
3a. Reduce personal safety risk	Relevant	The debris build-up behind the dam could entangle recreators.	Relative hazard potential: use dam characteristics and recreational use information
3b. Mitigate risk of failure	Not relevant	The dam did not have a hazard potential rating.
4a. Maximize swift water recreational opportunities	Relevant	The dam impeded the passage of swift water recreators during some conditions.	Accessible stream length : use connectivity methods
4b. Maximize flat water recreational opportunities	Not relevant	The dam did not impound a sufficient volume of water to support flat water recreation.
4c. Minimize adverse impacts to sites judged by the community, state, or Nation to be historically significant places	Relevant	The dam was over 50 years old, retained its original setting, location, and many of the original materials (including the original machinery in the powerhouse). These characteristics met criteria for listing in the National Register of Historic Places.	Register of historic places listing: determine eligibility for listing
4d. Minimize interruption to community sense of place	Unknown	Although there was not a clear public access point to the dam, it was accessible to the university community and recreators traveling from other access points.	Community preference: survey community
5a. Promote/maintain a population or community of focal taxa	Relevant	Consultation with other stakeholders revealed that the dam also posed a barrier to movement of the Altamaha shiner (Cyprinella xaenura), which is listed as threatened by the Georgia Department of Natural Resources.	Accessible stream length: use connectivity methods
5b. Promote/ maintain biodiversity	Unknown	The extent to which loss of connectivity or other environmental conditions caused by the dam constrained local biodiversity is unknown.	Current watershed, riparian, instream physical and/or chemical condition: assess severity of other stressors
5c. Prevent the spread of invasive species, disease, or undesirable hybridization	Not relevant	Removal had negligible chance of increasing risks of invasive species, diseases, or undesirable hybridization
5d. Reduce greenhouse gas emissions	Unknown	The effect of the dam on greenhouse gas emissions was unknown.	Estimate current contribution of the dam to greenhouse gases emissions: measure CO2 and CH4 emissions along a transect upstream to downstream of the dam to determine if there are elevated emissions in the impounded area
5e. Maintain or improve water quality	Unknown	The effect of the dam on water quality was unknown.	Current water quality: measure relevant parameters (e.g., temperature, dissolved oxygen) upstream to downstream of the dam to evaluate the impact of the dam on water quality
5f. Reduce stream geomorphic degradation	Not relevant	Negligible sediment storage and high background sediment transport rates indicated that dam removal had minimal potential effect on geomorphology

Figure 3. Photograph of White Dam, Athens, GA, USA. The star on the inset map represents the dam location

SUPPORT FOR MULTIPLE DAM DECISIONS

In a decision involving multiple dams, the decision process can depend on whether a single entity owns all of the dams considered for removal. Single owners are frequently infrastructure utilities or government agencies. We refer to the single owner context as a portfolio decision. In a portfolio decision, the decision-maker may know a great deal about their dams and primary objectives. In this case, the Dam Objectives and Metrics Selector application may be most useful in quantifying the co-benefits of a decision, for example to gain public support for the decision. In an initial prioritization of removals in a dam portfolio, the metrics may be more abstracted from the process which achieves the objective than the metrics used for a decision about a single structure or a decision among a few top alternatives. For example, for Objective 4a (maximizing swift water recreation), the decision analyst may choose to estimate accessible stream length under different removal scenarios using connectivity methods to identify a few desirable removal scenarios. Once a few scenarios have been identified, the analyst may choose to estimate the annualized monetary value of recreation under each scenario using stated or revealed preference methods with primary survey data (Black et al. 1998, Loomis 2002). The Dam Objectives and Metrics Selection application provides multiple metrics for most objectives to accommodate both stages of decisions.

If the dam portfolio owner provides infrastructure services with their dams, the removal decision may require metrics that are not directly related to the dams themselves but concern the feasibility of providing these services through other means. For example, a power utility prioritizing removals among their hydropower dams may include the additional power generation capacity from existing assets or the cost to develop new assets as metrics in their decision analysis. The Dam Objectives and Metrics Selector application does not provide these metrics because we assumed that utilities are best equipped to identify the appropriate feasibility metrics for their decision contexts.

Removal prioritizations are also developed among dams with different owners. These prioritizations are typically initiated by a non-owner entity whose mission is related to one or a few objectives like fish conservation or recreator safety. This entity often serves a facilitation role in identifying potentially beneficial removals and bringing together various stakeholders. We refer to these decisions as coalition decisions. We envision that the Dam Removal and Objective Selector application could be used to identify priority objectives in coalition decisions. Collaborations can stall at the stage of defining objectives because stakeholders are unable to articulate objectives from their values or recognize the coherence among priorities stated in slightly different ways (Gregory & Keeney 2012). This application may streamline the objectives definition process by providing language for objectives and their categories that stakeholders can use to locate their priorities and ensure that important objectives are not excluded. As in portfolio decisions, the application can also be used in coalition decisions to identify metrics appropriate to the decision stage.

OUTLOOK

Decision analyses often result in the selection of a suboptimal alternative early in the decision process due to failure to define all objectives or to select appropriate metrics and methods to parameterize those objectives (Hemming et al., 2022). By identifying common objectives, we aimed to facilitate critical thinking about management objectives. We anticipate that this tool may be particularly useful for facilitating discussion among stakeholders identifying project objectives. By outlining metrics, methods, data sources, and decision support tools associated with these objectives, we aimed to guide decision makers to relevant resources for systematically evaluating decision alternatives of dam removal, including the decision to retrofit, repair, partially remove, or do nothing. Using this application to identify objectives and select metrics/methods may reveal key uncertainties that require additional analysis. The application may also be useful to inform dam owners about the diverse objectives of dam removal, to identify opportunities to align benefits and diverse project funding sources, and to identify consistent categories of benefits to compile and report the consequences of multiple projects.

More broadly, we propose this process—identifying common objectives, metrics, methods, and decision frameworks—as a model to facilitate structured decision-making for other types of river infrastructure decisions (e.g., decisions about levees and navigation dredging). Historically, many forms of infrastructure were constructed to meet a single, primary objective (National Academies of Sciences, Engineering, and Medicine 2022). In a world facing the interacting crises of biodiversity loss, climate change, and infrastructure deterioration, we maintain that the single objective decision-making leads to less efficient outcomes, including missed opportunities to align important objectives.

ACKNOWLEDGEMENTS

This research was conducted as part of the Network for Engineering with Nature (N-EWN, https://n-ewn.org). This work was supported by the US Army Corps of Engineers Engineering With Nature® Initiative through Cooperative Ecosystem Studies Unit Agreement W912HZ-20-20031. The use of products or trade names does not represent an endorsement by either the authors or the N-EWN. Opinions expressed here are those of the authors and not necessarily those of the agencies they represent or the N-EWN.

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SUPPORTING INFORMATION

Elaboration of author contributed metrics/methods

Objective 2a. Change in water surface elevation relative to water intakes: A chief concern of municipalities regarding water supply is the elevation of their intake pipes, particularly during low-flow periods. For reservoirs where dam decommissioning is considered, it is imperative that the post-removal water surface elevation remains adequately high to maintain withdrawal through the current intake; otherwise, the cost of installing a new intake should be accounted for in the decision process. We recommend the use of an appropriate hydraulic model to estimate water surface elevation during low flow periods.

Objective 2a. Change in local average groundwater table depth: Twenty-five percent of water use in the US is from groundwater (Dieter et al., 2018), which is connected to reservoirs through surface water-groundwater exchange. In regions where this is a potential issue, dam decommissioning analyses should include groundwater modeling to evaluate how changes to stream and reservoir elevations propagate into the local aquifer to ensure that groundwater supply is not compromised.

Objective 2b. Users of dam power: The population served by dam-generated power provides an additional element for understanding its importance for service delivery.

Objective 2c. Users of in-stream dam navigation: The annualized number of vessels using a dam for navigation is an indicator of its importance for transportation.

Objective 2c. Users of terrestrial dam navigation: The annualized number of vehicle or axle crossings over a dam is an indicator of its importance for transportation.

Objective 2c. Proximity to nearest alternative stream-crossing infrastructure: An important consideration when evaluating decommissioning options is to determine the distance traffic must be re-routed to cross the stream if the road that a dam supports is removed. This is useful to couple with the metric for number of vehicles crossing the road over the dam to determine the complete effect of removing that road (e.g., if traffic must be re-routed, is the amount of traffic or re-routed distance sufficiently low?).

Objective 3a. Proportion of annual flows classified as dangerous submerged hydraulic jumps: Low head dams can be a significant hazard for dangerous submerged hydraulic jumps, but that hazard is dependent on flow conditions over the dam. Whereas the other method for this metric evaluated potential hazard based on simple dam characteristics, this method provides a more detailed assessment for how frequently these dangerous flows may occur. Combining flow frequency analysis with physical dam measurements to estimate this hazard helps decision makers quantify the prevalence of life-threatening flow conditions.

Objective 4a. Accessible stream length: Dams can impede swiftwater recreation by creating barriers to the movement of small craft like kayaks, canoes, and rafts. Connectivity methods can be used to estimate the changes to accessible areas for swiftwater recreation under different dam removal alternatives.

Objective 4b. Accessible flat-water area: Dams can create opportunities for flat-water recreation like motorized boating, swimming, and paddle boarding. Calculating accessible flat-water area can indicate opportunities for this type of recreation under different dam removal alternatives.

Objective 4c. Average community ranking of historical significance: Sites without formal historical recognition (e.g., National Register of Historic Places) may still be important to a community’s historic memory (Fox et al., 2016). Asking community members to rank their evaluation of a dam’s historical significance with other places in the community could indicate the relative importance of the dam to the community’s historic memory.

Objective 5e. Net water quality improvement: Dam decommissioning can have variable effects on the many aspects of water quality (e.g., temperature, dissolved oxygen, sediment, nutrients, etc.), so it is important to estimate the direction and magnitude of these changes. Generating a water quality model can inform a quantitative assessment of potential local and downstream benefits of dam decommissioning.

Objective 5f. Net change in channel width/depth ratio, slope: Dam decommissioning can release a volume of sediment that alters the local and downstream channel morphology, affecting flow characteristics and connection to the floodplain. Common methods to quantify the effects of bed and bank erosion are through channel slope and width-depth ratio. Substantial shifts in these parameters can trigger channel instability and impair stream and riparian function.

Objective 5f. Erosion potential: Erosion potential assesses the susceptibility of the stream to erode based on the amount of sediment available and the stream’s ability to move that sediment. This is imperative to understanding potential geomorphic channel change that may result from dam decommissioning.

Literature search details

For the systematic review of existing dam removal decision-support tools, we searched Web of Science on April 11, 2022, using the search strings “(multiobjective OR multi-objective OR multicriteria OR multi-criteria) AND dam removal” as well as “prioritiz* AND (dam removal OR dam decommissioning)”). From the 86 results, we identified 9 additional, relevant papers that were not captured in our original search. We also reviewed 13 river barrier prioritization tools identified by American Rivers (American Rivers, 2022). We included tools returned by these search parameters that concerned optimal siting for dam construction, as many criteria are shared between dam construction and removal decisions. We did not include tools in our analysis if they only included metrics associated with a single objective (16% of evaluated literature, e.g., Kocovsky et al., 2009) or solely concerned water infrastructure other than dams (13% of evaluated literature, e.g., González-Zeas et al., 2019).

Elaboration of application features

We constructed the Dam Objectives & Metrics Selector web application using R Shiny (Chang et al. 2022). The Guidance landing page hosts this manuscript to provide context on the intended use of this application. On the Objectives tab, the user can select from among the listed objectives, and the associated metrics and methods will be displayed in a table on the Metrics tab (Figure 4). Clicking on the Objectives Categories labels displays dialogue boxes containing justifications for each of the objectives in the selected category. Similarly, clicking the information icon associated with each of the methods in the table in the Metrics tab displays a dialogue box containing information about the data requirements and sources for that method as well as citations. Methods not derived from existing published literature are cited as “Authors” (Appendix A). The Tools tab displays citations for tools which include the user’s selected objectives. The final Feedback tab provides a space for users to alert the authors to additional resources to include in the application. By this mechanism, we envision the application as a dynamic community resource.